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In humans, the stomach is a highly acidic environment (maintained by the secretion of Hydrochloric acid) with peptidase digestive enzymes. In fact, the stomach's interior can secrete 2 to 3 liters of gastric fluid per day.
In ruminants, the stomach is a large multichambered organ that hosts symbiotic bacteria which produce enzymes required for the digestion of Cellulose from Plant matter. The partially digested plant matter passes through each of the stomach's chambers in sequence, being regurgitated and rechewed at least once in the process.
The stomach lies between the Esophagus and the first part of the Small intestine (the Duodenum). It is on the left side of the Abdominal cavity, the fundus of the stomach lying against the diaphragm. Lying beneath the stomach is the Pancreas, and the greater omentum hangs from the greater curvature.
It is divided into five sections, each of which have different cells and functions. The Gastric juice which is in the stomach has a pH of 1-3. On the pH scale, the larger the pH level, the less acidic a substance is. Sometimes gastric juice erodes the stomach wall or its layer of mucus, causing an Ulcer.
Starting inside the stomach (the lumen) going out, the first main layer is the mucosa. This consists of an Epithelium, the Lamina propria underneath, and a thin bit of Smooth muscle called the muscularis mucosa.
The submucosa lies under this and consists of fibrous Connective tissue, it separates the mucosa from the next layer, the muscularis externa. The muscularis in the stomach differs from other GI organs in that it has three layers of muscle instead of two. Under these muscle layers is the adventitia, layers of connective tissue continuous with the omenta.
The Epithelium of the stomach forms deep pits, called fundic or oxyntic glands. Different types of cells are at different locations down the pits. The cells at the base of these pits are chief cells, responsible for production of pepsinogen, an inactive precursor for Pepsin, which degrades proteins. The secretion of pepsinogen prevents self-digestion of the stomach cells.
Further up the pits, parietal cells produce Gastric acid, which kills most of the bacteria in food, stimulates hunger, and activates pepsinogen into pepsin. Near the top of the pits, closest to the contents of the stomach, there are Mucus producing cells called goblet cells that help protect the stomach from self-digestion.
The muscularis externa, as previously mentioned, is made up of three layers of Smooth muscle. The innermost layer is obliquely oriented, this is not seen in other parts of the digestive system, this layer is responsible for creating the motion that churns and physically breaks down the food. The next muscle layers are the circular and then the longituditinal, which are present as in other parts of the GI tract. The antrum has thicker muscle in its walls and performs more forceful contractions than the fundus. The pylorus is surrounded by a thick circular muscular wall which is normally tonically constricted forming a functional (if not anatomically discrete) pyloric Sphincter, which controls the movement of Chyme into the duodenum.
The hormone Gastrin causes an increase in the secretion of HCl, pepsinogen and Intrinsic factor from parietal cells in the stomach. It also causes increased motility in the stomach. Gastrin is released by G cells in the stomach to distenstion of the antrum, and digestive products. It is inhibited by a PH normally less than 4 (high acid), as well as the hormone Somatostatin.
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Cholecystokinin (CCK) has most effect on the gall bladder, but it also decreases gastric emptying. Similarly Secretin, produced in the small intestine, has most effects on the pancreas, but will also diminish acid secretion in the stomach.
Gastric inhibitory peptide (GIP) and Enteroglucagon decrease both gastric acid and motility.
Other than gastrin, these hormones all act to turn off the stomach action. This is in response to food products in the intestines, which have not yet been absorbed. The stomach only needs to push food into the small intestine when the intestine isn't busy. While the intestine is full and still digesting food, the stomach acts as a storage hopper for food.
This pattern is also present in the nervous control of the stomach.
| Digestive system |
| Mouth - Pharynx - Crop - Esophagus - Stomach - Pancreas - Gallbladder - Liver - Small intestine (Duodenum, Jejunum, Ileum) - Colon - Cecum - Rectum - Anus |
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